An image sensor provides a grid of pixels which may contain photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, and/or transfer transistors for recording intensity or brightness of light. The pixel responds to the light by accumulating photo-charges—the more light, the more the photo-charges. The charges can then be used by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD), a complimentary metal oxide semiconductor (CMOS) image sensor (CIS), an active-pixel sensor (APS), and a passive-pixel sensor.
When integrated with semiconductor devices to be applied to modern consumer electronic devices, size of image sensors is desired to be reduced to ensure mobility of the devices. For example, CIS is widely used in applications such as digital camera or mobile phone camera due to its miniaturized size. These devices utilize an array of pixels located in a substrate, including photodiodes and transistors that can absorb radiation projected toward the substrate, and convert the sensed radiation into electrical signals. However, the demand for even smaller image sensors has posed challenges such as heat control or durability. Measures to improve semiconductor image sensors are continuously being sought.